1. Field of the Invention
The present invention relates to an image forming apparatus and its controlling method.
2. Description of the Related Art
As an image forming unit for an electrophotographic printer, for example, an image forming unit 1100 as shown in FIG. 11 is known. FIG. 11 is a schematic diagram of the image forming unit 1100 that includes a photosensitive element 1101, a charging unit 1102, an exposing unit 1103, a developing unit 1104, a sheet conveying unit 1105, a transfer unit 1106, a cleaning unit 1107, and a neutralization unit 1108. The photosensitive element 1101 rotates in a direction indicated by an arrow “a” in FIG. 11, during which the surface thereof is uniformly charged by the charging unit 1102. The charged surface is irradiated with light corresponding to printing data by the exposing unit 1103, so that a latent image is formed on the surface of the photosensitive element 1101. Then, toner is adhered to the latent image by the developing unit 1104 to form a toner image on the surface of the photosensitive element 1101. The toner image is transferred onto a sheet conveyed by the sheet conveying unit 1105 in a direction indicated by an arrow “b” in FIG. 11. The sheet carrying the toner image is conveyed to a fixing unit (not shown), in which the toner image is fixed to the sheet. Toner remaining on the surface of the photosensitive element 1101 is removed by the cleaning unit 1107, and the neutralization unit 1108 neutralizes the surface of the photosensitive element 1101.
There is a demand for an image forming apparatus that includes such an image forming unit to enable restarting of printing promptly when a printing instruction was received while each unit in the image forming unit is in a stopping process. For example, a technology is disclosed in Japanese Patent Application Laid-open No. H06-143683, in which when an image forming apparatus is instructed to perform a stopping process, the image forming apparatus stands by for a certain period of time in a state of stopping only a developing unit. When a printing instruction is received during the standby, printing can be started only by restarting the developing unit. When the printing instruction is not received during the standby, the stopping process is performed also on the units other than the developing unit such as a transfer unit and a cleaning unit after the certain period of time.
However, in the above technology, the units other than the developing unit are driven uselessly for the certain period of time, so that the lifetime of the units may be shortened. Moreover, even when the printing instruction is received at the moment when the stopping process of each unit of the image forming unit is started, the stopping process of the developing unit is inevitably performed. Therefore, the time required for restarting printing cannot be shortened anymore.